Difluoromethane, known under the designation F32, is harmless to the ozone layer. It is therefore particularly advantageous for replacing CFCs. As a mixture with other hydrofluoroalkanes such as 1,1,1-trifluoroethane (F143a), 1,1,1,2-tetrafluoroethane (F134a) or pentafluoroethane (F125), it is intended especially to replace F22 (chlorodifluoromethane) and F502 (azeotropic mixture of F22 and of chloropentafluoroethane) in the field of refrigeration, of conditioned air and in other applications.
There are various known processes for the synthesis of F32. The hydrogenolysis of F12 (dichlorodifluoromethane) or of F22 (patents JP 60-01731 and EP 508 660) has the disadvantage of being generally not very selective and of producing worthless methane as by-product. It has recently been proposed to produce F32 by fluorination of bis(fluoromethyl) ether (patent EP 518 506).
It is also possible to produce F32 by fluorination of methylene chloride (F30) with anhydrous HF. Many patents describe this reaction, claiming the use of catalysts such as Cr.sub.2 O.sub.3, CrF.sub.3, AlF.sub.3, Cr/carbon, Ni/AlF.sub.3 etc.
The difficulty of this reaction lies in the stability of the catalyst, which tends either to coke rapidly or to crystallize. The problem becomes very tricky if it is intended to combine a high space time yield and a good selectivity while maintaining good stability of the catalyst.
To reduce this deactivation it has been proposed to employ specific catalysts such as a mechanical mixture of alumina and chromium oxide (patent GB 821 211). This patent gives an example for the fluorination of methylene chloride but the F32 space time yields obtained on this catalyst are low (&lt;200 g/h/l) and the cumulative duration of the tests is shorter than 5 hours.
More generally, during fluorination reactions, it is very often envisaged to inject oxygen or air continuously in order to lengthen the lifetime of the catalysts. Thus, patent JP 51-82206 claims the use of 0.001 to 1% of oxygen to maintain the activity of catalysts prepared from chromium oxide. The examples in this patent relate only to reactions of fluorination of perhalogenated molecules (CCl.sub.4, C.sub.2 Cl.sub.3 F.sub.3).
The major disadvantage of this process is the appearance of a Deacon reaction. In fact, chromium oxide, well known as a fluorination catalyst, is also a good catalyst for the oxidation of HCl (patents U.S. Pat. No. 4,803,065 and U.S. Pat. No. 4,822,589). The oxygen introduced during the fluorination reaction reacts with the HCl formed to produce water and chlorine. Because of corrosion problems, the presence of water is particularly undesirable in a fluorination process.
Continuous introduction of a small quantity of chlorine has already been proposed in patent JP 49-134612, in order to stabilize the activity of the catalysts employed for the disproportionation of perhalogenated molecules; in this case the use of chlorine does not result in a decrease in the selectivity.
More recently the use of chlorine as an inhibitor of deactivation has also been described in the case of the fluorination of CF.sub.3 CH.sub.2 Cl (US Statutory Invention Registration H1129). The examples which are presented clearly show that the use of chlorine makes it possible to maintain a stable space time yield of CF.sub.3 CH.sub.2 F (F134a). On the other hand, no indication is given as to the effect of chlorine on the selectivity of the reaction.
However, in the case of hydrogenated molecules and in particular in the case of the fluorination of CF.sub.3 CH.sub.2 Cl (F133a), the presence of chlorination reactions has been demonstrated, resulting in the formation of worthless by-products. Thus, in the case of the fluorination of F133a, by-products of the F120 series (C.sub.2 HCl.sub.n F.sub.5-n) are chiefly formed.
Given that the Deacon reaction produces chlorine, this loss in selectivity is also observed during the fluorination of hydrogenated molecules in the presence of oxygen on chromium catalysts. This is why some patents (see, for example, patent EP 546 883) have claimed the preparation of specific catalysts limiting the oxidation of HCl and the by-production of chlorine.
It could be assumed that the behaviour of methylene chloride would be similar to that of F133a, and this would make the use of chlorine not very advantageous for maintaining the activity of the catalyst. However, it has been surprising to find that, in the case of the methylene chloride fluorination, even with relatively high contents (Cl.sub.2 /F30=3 mol %), chlorine undergoes very little reaction with the compounds of the F30 series (CH.sub.2 Cl.sub.n F.sub.2-n), and this allows it to be employed without any significant decrease in the selectivity of the reaction.
In addition, in the abovementioned patent JP 51-82206 it is indicated that oxygen enables the catalyst activity to be maintained even in concentrations that are lower than that employed with chlorine. However, it has been found that, during the fluorination of methylene chloride, the continuous introduction of chlorine is, at an equal concentration, a more effective means than the addition of oxygen in order to stabilize the activity of the catalysts. In fact, in high space time yield conditions, oxygen addition is not sufficient to maintain the activity of the catalysts even at high temperature, whereas the addition of chlorine enables their lifetime to be significantly lengthened from a temperature of 250.degree. C. upwards and therefore the fluorination of methylene chloride to be carried out in a temperature range in which an irreversible deactivation of the catalyst by crystallization is not very probable.